This invention relates generally to adaptive optics systems and, more particularly, to techniques for measuring relative phase tilt of an optical beam in two dimensions across a planar array. Adaptive optics systems employ adjustable optical elements, such as deformable mirrors, to compensate for aberrations in an optical beam. Aberrations may be caused, for example, by propagation of the beam through the atmosphere. In a typical adaptive optics system, the aberrated beam is reflected from a deformable mirror having many small elements that are adjustable in position, using a separate actuator associated with each movable mirror element. Part of the reflected beam is split off and directed to impinge on a sensor array, which provides measurements indicative of wavefront distortion in the reflected beam. The wavefront distortion measurements are then fed back to the deformable mirror to provide continuous corrections by appropriately moving the mirror elements.
The principal components of a wavefront sensor array are an array of small lenses, referred to as a lenslet array, and camera having an array of sensor elements. This configuration is referred to as the Shack-Hartmann wavefront sensor. The lenslet array, when disposed in the path of the beam, produces multiple elemental portions of the beam, referred to as subapertured portions. Each lenslet in the lenslet array processes a subaperture of the whole beam. The lenslet array typically has each of its lenslets positioned on a square grid, although the whole array may not be square, and is usually a polygon shape conforming approximately to the outline of a circular beam. The camera sensor elements are usually square or rectangular in shape and are also usually arranged in a square or rectangular grid pattern, angularly aligned to be parallel with the lenslet grid pattern.
There are two configurations of the Shack-Hartmann sensor in the prior art. In one configuration, each subaperture or lenslet is centered equidistantly with respect to the positions of four adjacent mirror actuators. Moreover, the lenslet is centered over a group of four camera sensor elements, referred to as a quad-cell. A measure of phase tilt in one direction is derived from the difference between signal outputs from each of two pairs of the cells in the quad-cell. For example, if the cells and their output signals in a quad-cell are labeled A, B, C and D, where cells A and B are aligned in the X-axis direction and cells A and C are aligned in the Y-axis direction, the X-axis tilt is determined from (A+C)-(B+D)!/A+B+C+D!. Similarly, the Y-axis tilt is determined from (A+B)-(C+D)!/A+B+C+D!.
The principal drawback of this approach is that it would be preferable to measure the local X-axis tilt between two adjacent actuators on a common X axis, and to measure the Y-axis tilt between two adjacent actuators on a common Y-axis. An alternate configuration of the prior art avoids this disadvantage but, unfortunately, introduces another difficulty. In this alternate configuration, two lenslet arrays and two arrays of detectors are needed, one to measure the X-axis tilt and the other to measure the Y-axis tilt. The lenslet array for measuring the X-axis tilt is positioned such that each subaperture or lenslet is positioned midway between the grid positions corresponding to two adjacent actuators on a common X axis. Similarly, the other lenslet array is positioned such that each subaperture or lenslet is positioned midway between the grid positions corresponding to two adjacent actuators on a common Y axis. For X-axis tilt measurement, a camera sensor array provides a bi-cell, i.e. two adjacent sensor cells located between the adjacent actuators in the common X axis. Light from the lenslet is, therefore, normally positioned over the boundary between the two cells in the bi-cell, and any tilt in the X-axis direction is determined from the difference in output signals from the two cells in the bi-cell. Similarly, for Y-axis tilt measurement, a separate camera sensor array provides a bi-cell between two grid positions corresponding the adjacent actuators in the common Y axis. Thus, Y-axis tilt is derived from the difference between output signals from two cells in the bi-cell. The difficulty introduced by this configuration is that two lenslet arrays and two cameras are required. The two lenslet arrays will necessarily interfere with each other physically if they are placed in a single beam being analyzed. Use of beam splitters to provide two separate beams for analysis by the two lenslet arrays and cameras introduces further complications in terms of alignment.
Ideally, what is needed is a technique for measuring phase tilt by sensor elements positioned between adjacent actuators in both the X axis and adjacent actuators in the Y axis, but without the difficulties of physical interference or alignment posed by the alternate configuration of the prior art. The present invention fulfills this need.